In a cellular mobile radio communications system, it is of utmost importance to keep the interference between cells at a minimum, in order to minimise disturbances on a radio connection. This can be achieved in a cell by using frequencies that are well separated, in the frequency domain, from the frequencies used in nearby cells and by keeping the frequency re-use distance sufficiently large. However, since the frequency span available for mobile radio communication is limited, a large frequency re-use distance means low capacity of the mobile radio communication system. Hence, it is desirable to find ways of reducing the frequency re-use distance, while keeping interference between cells at a minimum.
In order to achieve a reduced frequency re-use distance while reducing interference, beam-forming antennas may be used for the dedicated channels, so that a signal carrying a dedicated channel, intended for a particular user equipment, is transmitted by a beam-forming antenna transmitting a beam in the direction in which the user equipment is located in relation to the transmitting radio base station. The beam-forming may be achieved by use of fixed beam-forming antennas, for which the shape and direction of the transmitted beam is fixed or quasi-fixed, or by adaptive beam-forming antennas, for which the direction and/or shape of the beam is adjusted in response to a signal received from the user equipment for which the signal transmitted from the beam-forming antenna is intended.
By using beam-forming antennas for the dedicated channels, the interference on nearby cells caused by the transmission of dedicated channels within a cell can be reduced, as well as the energy consumption of a radio base station. However, for the common channels, such as for example a random access channel, broadcast channels and pilot channels, the situation is different. A common channel should be detectable by all user equipments in a cell, including those user equipments that have not yet performed random access. Hence, since the location of, or even the existence of, potential receivers of a common channel signal is not know, the common channel signal cannot be transmitted in a particular direction corresponding to the direction of the location of the intended receiver.